1. Field of the Invention
The present invention relates to memory devices and methods for manufacturing the same and, more particularly, to phase change memory devices and methods for manufacturing the same.
2. Description of the Related Art
Electrically writable and erasable phase change materials are used in semiconductor memory devices. The phase change material can be electrically switched by a change between an amorphous state and a crystal state. See, for example, U.S. Pat. Nos. 3,271,591 and 3,530,441 to Ovshinsky. The phase change material can be switched in an incremental step, reflecting a change of a localized order, in order to provide a “gray scale” represented by various conditions of the localized order from the amorphous state to the crystal state. The phase change material also can be switched between two structural states of the localized order, that is, roughly amorphous and roughly crystal, thereby storing and retrieving binary information.
A semiconductor memory device that uses a phase change material may be referred to as a phase change memory device or a phase-change Random Access Memory (RAM). As the phase change material, a chalcogenide material is commonly used. Accordingly, phase change memories also may be referred to as chalcogenide memories. Other phase change materials also may be used. In order to initialize a detectable phase change of a localized arrangement, a relatively high energy may be used. That is, in order to obtain detectable changes of chemical and electronic bonding structures of the chalcogenide material, relatively high energy may be provided.
FIG. 1A is a cross-sectional view illustrating a schematic structure of a phase change memory device disclosed in US published application No. 2001/0049189 to Zahorik, entitled to “Small Electrode for Chalcogenide Memories”. 
As shown in FIG. 1A, a conventional phase change memory device comprises a lower electrode 12 formed in a contact hole C within an interlayer insulating layer 11 covering a semiconductor substrate 10 and connected to the semiconductor substrate 10, a phase change layer 13 stacked on the lower electrode 12, a conductive adhesive film 14, and an upper electrode 15.
FIG. 1A shows a reset state where a part of the phase change layer 13 is changed into an amorphous state A, and FIG. 1B shows a set state where the phase change layer 13 is changed into a crystal state B. Reference numeral “A” in FIG. 1A represents that the phase change layer 13 is changed into the amorphous state, and reference numeral “B” in FIG. 1B represents that the phase change layer 13 is changed into the crystal state.
In order to increase current density applied to the phase change layer 13, the lower electrode 12 is formed within the contact hole C, to reduce the cross-sectional area, as shown in FIGS. 1A and 1B.
The conventional phase change memory device writes and reads data using the resistance change under the state of set or reset of the phase change layer 13. To this end, a relatively large programming current, for example several mA, may be used. The programming current generally is proportional to an area of the phase change layer 13 that contacts the lower electrode 12. Moreover, it may take hundreds of nanoseconds to several microseconds for the conventional phase change memory device to crystallize an amorphous region, which may affect the speed of the device. The crystallization speed is also generally proportional to an area of the amorphous region that contacts with the lower electrode. Therefore, in order to reduce the programming current and shorten the crystallization time, it may be desirable to reduce contact area between the phase change layer 13 and the lower electrode 12.
However, a size of a contact hole C that determines the contact area between the lower electrode 12 and the phase change layer 13 generally depends on a photolithography process, and it may be difficult to reduce the area of the lower electrode below a certain level.